Issue

A Floating Window (Group with at least 2 tool windows) does not hide correctly.

The framework hides only the toolwindows but leaves floating window empty.
There is a NullReference Exception + Crash when user tries to drag the flowting tool window.

Steps to verify (using the MLib_Test client)

1. Drag 2 tool windows (LayoutAnchorable) into one floating window
   

2. Close MLib_Test Application to Save the Layout

3. Restart MLib_Test Application to Reload the Layout

4. Click Hide (button) of the floating window
   

Bug

• The empty floating window is still visible
• Dragging the empty floating window leads to a crash
  AvalonDock.Layout.txt

Crash is reproducable with this layout but somehow not with above steps anymore(?):